The present invention relates to the switching of electric power through avalanche action throughout the volume of a block of semiconductor material.
One method of switching electrical current is to provide two contacts to a block of semiconductor material. The semiconductor material is doped so that it acts as an insulator. When light from a laser is directed on the semiconductor material, photoconduction occurs which causes the semiconductor block to conduct. Typically, a powerful desk top laser is required to generate the required amount of photoconduction for significant amounts of current. A discussion of switches of this type is contained in U.S. Pat. No. 4,438,331 to Davis. Davis shows a switch which can be triggered with a laser diode.
An alternate method using avalanche breakdown is discussed in U.S. Pat. No. 4,347,437 to Mourou. Mourou applies a high voltage across the semiconductor material which is less than the voltage which would cause avalanche breakdown. Avalanche breakdown occurs when the charge carriers (i.e., electrons and holes) have sufficient kinetic energy due to the applied field so that when they collide with an atom they knock loose one electron/hole pair which in turn create further electron/hold pairs, thereby creating an exponentially increasing current flow. Mourou uses the high voltage to bias the semiconductor material below the voltage required for avalanche breakdown. A laser is then used to generate "seed" carriers which initiate the avalanche process. This builds to full avalanche conduction over a period of a few hundred picoseconds to more than one nanosecond.
Although Mourou does not set forth the details of the avalanche breakdown, it is believed the avalanche in his device proceeds with the creation of one or more "streamers" of conduction through the semiconductor material between the contacts, with the streamers increasing in diameter until the entire semiconductor is conducting. A streamer is a thin line of conduction between the contacts which may start at microscopic irregularities of the contact or the semiconductor material. The creation of such streamers would limit the useful life of the switches because of the high stress placed on the areas in which the streamers first occur.
The Mourou device requires that the semiconductor material is cooled to a cryogenic temperature to prevent thermally-induced carriers from starting conduction when the voltage is applied and before the laser is turned on. Mourou states that a laser diode with 3 nano-joules produces a megawatt of power using this method.